The present invention relates to a group III nitride crystal and crystal growth process and crystal growth apparatus of a group III nitride.
At present, the device of InGaAlN system (group III nitride) used for purple, blue or green optical source is mostly produced by a crystal growth process such as MO-CVD process (metal-organic chemical vapor deposition process), MBE process (the molecular beam epitaxy process), and the like, conducted on a sapphire or SiC substrate. In the case sapphire or SiC is used as the substrate, however, there arises problems such as increased crystal defects caused by the difference of thermal expansion coefficient or lattice constant with regard to the group III nitride. Thus, there have been various difficulties in a light-emitting device that uses such a group III nitride crystal such as poor device characteristics, difficulty in increasing the lifetime, large operational power, and the like.
In the case of using a sapphire substrate, which is an insulation substrate, it is impossible to take out the electrode through the substrate as is practiced in conventional light-emitting devices, and it becomes necessary to take out the electrode from the surface side of the nitride semiconductor formed by a crystal growth process. As a result, there arise problems such as increased device area and increased cost. Also, chip separation by cleaving has been difficult in a group III nitride semiconductor device produced on a sapphire substrate, and it has been difficult to form the cavity edge surface needed in a the laser diode (LD) by conducting a cleavage process. Because of this, it is practiced presently to form the cavity edge surface by conducting a dry etching process. Alternatively, the cavity edge surface is formed in the process somewhat similar to a cleavage process after polishing the sapphire substrate to the thickness of 100 μm or less. In this case, too, it is impossible conduct the formation of cavity edge surface and the separation of the chips process simultaneously as is practiced in a conventional LD, and problems such as complexity of the process and increased cost are not avoidable.
In order to solve these problems, there is a proposal to reduce the crystal defect by conducting selective lateral growth or other process of the group III nitride semiconductor film on a sapphire substrate. According to this method, it has become possible to reduce the crystal defects as compared with the case in which such a selective lateral growth of the GaN film is not conducted on the sapphire substrate. However, the problem of insulating nature and difficulty of cleavage process mentioned before caused by the sapphire substrate is still remaining. Further, there arise problems which lead to cost increase such as complication of the process and warp of the substrate caused by as a result of using different materials in the case of using a sapphire substrate together with a GaN thin film.
In order to solve such a problem, the use of a bulk GaN substrate, which is the same material grown on the substrate, is deemed most appropriate. Therefore, investigations are being made on the crystal growth of bulk GaN by using a gas phase growth process or a liquid phase growth process. However, there has been no report about high quality GaN substrate successfully formed to have a practical size.
For example there is a proposal in Conventional Technology 1 to grow a bulk crystal of GaN and use the bulk crystal of GaN thus grown as the substrate for homoepitaxial growth. According to this proposal, the GaN bulk crystal is grown from a Ga melt in a nitrogen ambient of ultra high pressure such as several ten kilobars at the high temperature of 1400–1700° C.
In this case, it becomes possible to grow group III nitride semiconductor films needed for a device by using the GaN bulk substrate formed by a bulk growth process. Thus, it becomes possible to realize a GaN substrate without complicating the process. However, there is a problem in such an approach that crystal growth under high temperature and high pressure is necessary while the reaction vessel enduring such a high pressure and high temperature process condition is extremely expensive. Also, there is a problem in that the crystals grown with this method form a plate-like crystal having a C principal surface of about 1 cm diameter, while the thickness thereof is only 20–30 μm at the maximum, and the crystal easily undergoes cracking or breaking during the device production process.
As an alternative method of realizing a GaN substrate, there is proposed in Conventional Technology 2 a growth process of a GaN crystal that uses Na as a flux. In this method, sodium azide (NaN3) and metallic Ga are confined in a reaction vessel of stainless steel (inner diameter=7.5 mm, length=100 mm) together with a nitrogen gas as the source materials, wherein the GaN crystal is grown by holding the reaction vessel at the temperature of 600–800° C. for 24–100 hours.
In the case of Conventional Technology 2, the crystal growth is possible at a comparatively low temperature of 600–800° C. Further, a comparatively low pressure of 100 kg/cm2 or less is used inside the vessel. Thus, this technology enables crystal growth under more practical growth condition. However, there is a problem in this method that the size of the crystal obtained is very small, not reaching 1 mm.
So far, the inventors of the present invention have made inventions shown in Conventional Technology 3, Conventional Technology 4, Conventional Technology 5, and Conventional Technology 6.
Here, it should be noted that Conventional Technology 3 discloses the technology of supplying a group V source material stably. Also, Conventional Technology 4 discloses the method of conducting crystal growth by using a seed crystal. Further, Conventional Technology 5 discloses growth of a group III nitride crystal by supplying group III source material to from a compound of a group III element and an alkali metal. Further, Conventional Technology 6 discloses growth of a group III nitride crystal of cubic crystal system.
In such crystal growth processes, it should be noted that the growth of the group III nitride crystal is caused by supplying a substance containing nitrogen to a melt mixture containing the group III element and alkali metal from outside of the reaction vessel.
Below, the invention of Conventional Technology 3 will be explained with reference to the drawings. FIG. 1 is a diagram showing an example of the construction of the crystal growth apparatus of Conventional Technology 3. Referring to FIG. 1, a melt mixture 102 of a group III element (Ga (gallium) for example) and a flux (metallic Na or a compound containing Na (sodium azide, and the like)) is accommodated into the reaction vessel 101. Further, heating device 105 capable of controlling the temperature to the temperature in which crystal growth becomes possible is provided to the reaction vessel 101.
Further, a seed crystal 103 (GaN crystal for example) is provided so as to make a contact with the gas-liquid interface 113 defining the boundary of the gas and the melt 102 inside the reaction vessel 101.
It should be noted that a nitrogen gas is used for the nitrogen source material. In order to supply the nitrogen gas into the reaction vessel 101, there is provided a first gas supply device 120 outside the reaction vessel 101, wherein the first gas supply device 120 is formed of a first cylinder 110 for storing the gaseous nitrogen source material and a first valve 111, and the nitrogen gas is supplied to reaction vessel 101 from the first cylinder 110 of the first gas supply device 120 filled with the nitrogen gas and located outside of the reaction vessel 101 by way of the nitrogen supply line 106.
In order to adjust the nitrogen pressure, there is provided a pressure adjustment system in the midway of the nitrogen supply line 106, wherein the pressure adjustment system of the nitrogen gas is formed of a pressure sensor 107 and a pressure adjustment valve 108 and the pressure information measured with pressure sensor 107 is transmitted to the pressure adjustment valve 108 through a cable 109. With this, the pressure adjustment is achieved by the pressure adjustment valve 108. It should be noted that the nitrogen pressure inside the reaction vessel 101 can be set to a desired value.
It should be noted that the nitrogen gas pressure inside the first cylinder 110 filled with the nitrogen gas set to be equal to or larger than the pressure inside the reaction vessel 101 at the time the group III nitride the crystal (GaN for example) is grown.
Thus, the nitrogen gas used as the nitrogen source material is supplied from the first cylinder 110 to the reaction vessel 101 after the gas pressure being adjusted with the pressure adjustment system.
By using the crystal growth apparatus of such a construction, there is caused growth of the group III nitride crystal (GaN crystal for example) under the condition, such as growth temperature, nitrogen pressure, Na quantity, and the like, set so as to enable crystal growth starting from the seed crystal 103, and the size of the group III nitride crystal is increased with time while using the melt 102, in which there are contained the alkali metal (Na for example), the group III element (Ga (gallium) for example) and the nitrogen gas supplied from outside as the source material.
Thus, in the invention of Conventional Technology 3, the nitrogen gas is supplied from the outside as a nitrogen source material in the state in which there is sufficient group III element and alkali metal (Na for example), and with this, a continual growth of the group III nitride crystal (GaN crystal) becomes possible, and it becomes possible to grow the group III nitride crystal (GaN crystal) to a desired size.
Similarly, a continual growth of the group III nitride crystal (GaN crystal) is carried out also in the inventions of Conventional Technology 4, Conventional Technology 5 and Conventional Technology 6, by supplying the nitrogen source material from the outside of the reaction vessel.
However, in any of these inventions in Conventional technologies 3, 4, 5 and 6, there have been a tendency in which a polycrystalline aggregate of group III nitride covers the melt mixture surface. When this takes place, the source material is depleted due to the consumption of the group III element in the melt mixture for the formation of the polycrystalline aggregate, and the desired large crystal is not obtained. Thus, there has been a problem in that it not possible to grow a crystal of desired size in the vicinity of the melt mixture surface.    Conventional Technology 1: Journal of Crystal Growth, vol.189/190, 153–158 (1998).    Conventional Technology 2: Chemistry of Materials, vol.19, 413–416, (1997).    Conventional Technology 3: Japanese Laid Open Patent Application 2001-64097.    Conventional Technology 4: Japanese Laid Open Patent Application 2001-64098.    Conventional Technology 5: Japanese Laid Open Patent Application 2001-102316.    Conventional Technology 6: Japanese Laid Open Patent Application 2001-119103.